The present invention generally relates to fault notifying systems, and more particularly to a fault notifying system for notifying a fault in an asynchronous transfer mode (ATM) communication system.
FIG. 1 shows an example of a conventional synchronous transfer mode (STM) communication system. For example, data terminal equipments (DTEs) 101.sub.1 through 101.sub.3 are coupled to a time division multiplexer (TDM) 102, and DTEs 105.sub.1 through 105.sub.3 are coupled to a TDM 104. The TDMs 102 and 104 are coupled via a STM network 103.
FIG. 2 shows an example of the general construction of the TDM 102 (or 104). The TDM 102 (or 104) includes a terminal interface 201 which is coupled to the DTE 201 (or 105), a multiplex controller 202, and a network interface 203 which is coupled to the STM network 103. In FIG. 2, a white circle indicates a reception and a black circle indicates a transmission.
If a circuit failure is generated, this failure is detected at the network interface 203 by detecting a received error code (REC) from the loss of circuit (line) synchronization, and an alarm indication signal (AIS) is supplied to the multiplex controller 202. This AIS has a signal format in which the bits are all "1" so as to cause synchronization error. The multiplex controller 202 supplies a data circuit-terminating equipment (DCE) not ready (DNR) code to the terminal interface 201 in response to the AIS. For example, the DNR code has a signal format in which a frame bit F is provided at the head and the bits are all "0". The terminal interface 201 notifies the DTE 101 of the detected failure by making RD=1, R=0, and CD, I=OFF. The signals RD and CD are prescribed under the CCITT Recommendation X-Series, and the signals R and I are prescribed under the CCITT Recommendation V-Series.
The detected failure is notified in the above described manner in the case of the STM network. However, in the case of the ATM network, the conventional means used to notify the detected failure cannot be employed, particularly because predetermined ATM cells are used for the communication via the ATM network. Hence, there is a demand to realize a system for efficiently and positively notifying the detected failure in the ATM network.
On the other hand, in the conventional STM communication system, the TDM fixedly assigns a band to each DTE, and the band assignment is inefficient if the DTE is unused, for example. Accordingly, it is desirable to improve the multiplexing efficiency of the TDM if applied to the ATM communication system.